Analog-to-digital converters have seen increased use in recent years due to the advances in digital signal processing and the increased use of digital transmission systems. Typically, analog-to-digital converters comprise circuitry for receiving an analog input signal and outputting a digital value that is proportional to the analog input signal. This digital output value can either be a parallel word or a serial digital bit string. There are many types of analog-to-digital conversion schemes such as voltage-to-frequency converters, charge redistribution, delta modulation, etc. Each of these techniques has advantages and disadvantages associated therewith.
One type of digital-to-analog converter that has seen increased use in recent years is that utilizing delta-sigma modulation wherein an analog voltage is input to a delta-sigma modulator and the output thereof filtered to remove noise. The delta-sigma modulator is of the type which converts an analog input to a digital pulse string having an average amplitude over time proportional to the analog input. One type of delta-sigma pulse modulator is described in U.S. Pat. No. 4,542,354, issued September 17, 1985 to Robinton, et al. Delta-sigma modulation provides for high accuracy and wide dynamic range as compared to earlier delta modulation techniques. The delta-sigma type modulation is sometimes referred to as an oversampled converter architecture which is immune from some of the earlier undesirable second order effects of delta modulation.
There are two key components of a delta-sigma analog-to-digital converter, the analog modulator and the digital filter. The analog modulator oversamples the analog input and produces a low resolution digital output. However, with any analog-to-digital converter, there are a number of noise sources that are inherent to any analog modulator design. In a delta-sigma modulator, there are output stage noise sources and input stage noise sources, the output noise sources normally being dominated by quantization noise and the input noise sources resulting from DC offset and 1/f noise. The quantization noise at low frequencies is relatively low with the largest portion thereof existing at higher frequencies. This higher frequency portion noise can be filtered out by a digital domain low-pass filter. However, the low frequency DC offset and 1/f noise cannot be filtered out by a low-pass filter, and, as such, this noise will be passed through the filter with the signal information. There therefore exists a need for an analog modulator that minimizes the low-frequency noise.